The present invention relates generally to magnetic random access memory devices, and, more particularly, to a method and apparatus for current sense amplifier calibration in MRAM devices.
Magnetic (or magneto-resistive) random access memory (MRAM) is a non-volatile random access memory technology that could potentially replace the dynamic random access memory (DRAM) as the standard memory for computing devices. The use of MRAM as a non-volatile RAM will eventually allow for “instant on” systems that come to life as soon as the system is turned on, thus saving the amount of time needed for a conventional PC, for example, to transfer boot data from a hard disk drive to volatile DRAM during system power up.
A magnetic memory element (also referred to as a tunneling magneto-resistive, or TMR device) includes a structure having ferromagnetic layers separated by a non-magnetic layer (barrier), and arranged into a magnetic tunnel junction (MTJ). Digital information is stored and represented in the memory element as directions of magnetization vectors in the magnetic layers. More specifically, the magnetic moment of one magnetic layer (also referred to as a reference layer) is fixed or pinned, while the magnetic moment of the other magnetic layer (also referred to as a “free” layer) may be switched between the same direction and the opposite direction with respect to the fixed magnetization direction of the reference layer. The orientations of the magnetic moment of the free layer are also known as “parallel” and “antiparallel” states, wherein a parallel state refers to the same magnetic alignment of the free and reference layers, while an antiparallel state refers to opposing magnetic alignments therebetween.
Depending upon the magnetic state of the free layer (parallel or antiparallel), the magnetic memory element exhibits two different resistance values in response to a voltage applied across the tunnel junction barrier. The particular resistance of the TMR device thus reflects the magnetization state of the free layer, wherein resistance is “low” when the magnetization is parallel, and “high” when the magnetization is antiparallel. Accordingly, a detection of changes in resistance allows a MRAM device to provide information stored in the magnetic memory element (i.e., a read operation).
In a 1-transistor, 1-MTJ type MRAM device, a column selector circuit is used to connect the device's regular (data) bit lines and reference bit lines with the input of a current sense amplifier (SA). During sensing, the SA maintains the voltage at the selected bitlines constant, and measures the current at the selected bitlines/reference bitlines. The magnitude of current at the selected bitlines is dependent on the programming state “0” or “1” of the selected memory cells. In order to keep the voltage at the selected bitlines approximately constant, a bitline voltage clamp is used. A comparator within the sense amplifier compares the voltage at a load device of the data cell being read with the voltage at a load device(s) of a reference bitline (or bitlines) and creates a digital output signal dependent on the programming state of the selected memory cell.
Unfortunately, a basic shortcoming of sense amplifiers, such as those used in MRAM devices, is the input offset current (or voltage) created by individual device mismatch within the sense amplifier. The device mismatch in turn creates a slight imbalance in the comparator input, with respect to the differential between the data signal and the reference signal, thus slightly biasing the sense amplifier toward a “1” state or a “0” state. Accordingly, it would be desirable to be able to compensate for any such imbalance/mismatch in the sense amplifier devices in order to improve the yield that would otherwise be lost due to the lack of signal margin.